The present invention relates to a process for the separation of mixtures comprising optionally azeotropic or pseudoazeotropic compositions. which contain hydrogen fluoride and 1,1,1,3,3-pentafluorobutane; and to a process for the synthesis of 1,1,1,3,3-pentafluorobutane.
1,1,1,3,3-Pentafluorobutane (HFC-365mfc) may be prepared by reaction of a suitable chloro precursor with hydrogen fluoride, as described for example in patent application EP-A1-0699649 in the name of SOLVAY. In such a process, on leaving the hydrofluorination reactor, the reaction product mixture contains, besides the desired 1,1,1,3,3-pentafluorobutane, hydrogen chloride originating from the elimination of the chlorine atom(s) in the starting chloro precursor, hydrogen fluoride and optionally inert diluents, as well as small amounts of various intermediates or by-products. Given that the process is usually performed with an excess of hydrogen fluoride relative to the chloro precursor, unconverted hydrogen fluoride usually remains in the reaction product mixture.
It is, desirable to have processes which make it possible to separate the 1,1,1,3,3-pentafluorobutane efficiently from the reaction product mixture. At the same time, it is desirable to ensure good use of raw materials, in particular of the hydrogen fluoride.
Consequently, the invention relates to a process for the separation of at least one constituent from a mixture comprising 1,1,1,3,3-pentafluorobutane and hydrogen fluoride, according to which the mixture is subjected to at least two distillations, the first distillation being carried out at a first pressure and the second distillation being carried out at a second pressure which is different from the first, and at least one fraction enriched in 1,1,1,3,3-pentafluorobutane and at least one fraction enriched in hydrogen fluoride are recovered.
It has been found, surprisingly, that the process according to the invention makes it possible to isolate 1,1,1,3,3-pentafluorobutane and hydrogen fluoride which are essentially pure, in particular from mixtures comprising 1,1,1,3,3-pentafluorobutane and hydrogen fluoride in proportions at which they form an azeotrope or a pseudoazeotrope.
The process according to the invention also makes it possible to isolate azeotropic fractions at a given pressure which are enriched either in hydrogen fluoride or in 1,1,1,3,3-pentafluorobutane.
In a variant of the process according to the invention, the first distillation is carried out at a higher pressure and the second distillation is carried out at a lower pressure.
In another variant of the process according to the invention, the first distillation is carried out at a lower pressure and the second distillation is carried out at a higher pressure.
The lower pressure to be used in the process according to the invention is generally at least 0.5 bar. Often this pressure is at least 0.9 bar. Generally the lower pressure is at most 3 bar. Often this pressure is at most 2.5 bar. Preferably the lower pressure is 1 to 2 bar.
The temperature at which the distillation at lower pressure is carried out is generally at least 0xc2x0 C. The temperature is often at least 10xc2x0 C. The temperature is generally at most 60xc2x0 C. Often, the temperature is at most 40xc2x0 C. Preferably, the temperature at which the distillation at lower pressure is carried out is 15 to 30xc2x0 C.
The higher pressure to be used in the process according to the invention is generally at least 6 bar. Often this pressure is at least 7 bar. Generally the higher pressure is at most 12 bar. Often this pressure is at most 11 bar. Preferably the higher pressure is 8 to 10 bar.
The temperature at which the distillation at higher pressure is carried out is generally at least 70xc2x0 C. The temperature is often at least 80xc2x0 C. The temperature is generally at most 150xc2x0 C. Often, the temperature is at most 120xc2x0 C. Preferably, the temperature at which the distillation at higher pressure is carried out is 80 to 100xc2x0 C.
The 1,1,1,3,3-pentafluorobutane content in the fraction enriched in 1,1,1,3,3-pentafluorobutane is often at least 50 mol %. More often it is at least 75 mol %. In a preferred variant, an enriched fraction consisting essentially of 1,1,1,3,3-pentafluorobutane is recovered, for example with a 1,1,1,3,3-pentafluorobutane content of at least 99 mol %.
The hydrogen fluoride content in the fraction enriched in hydrogen fluoride may be at least 99 mol %. It may be at least 99.5 mol %. In a variant, an enriched fraction consisting essentially of hydrogen fluoride is recovered.
Techniques and equipment which can be used to carry out the distillations in the process according to the invention are well known.
The mixture to which the process of separation according to the invention is applied may be, for example, a reaction mixture which is derived from a process of synthesis of 1,1,1,3,3-pentafluorobutane by hydrofluorination of a chloro precursor such as for example the process mentioned above. The process is often applied to mixtures comprising hydrogen fluoride and 1,1,1,3,3-pentafluorobutane in proportions at which they form an azeotrope or a pseudoazeotrope.
In a variant of the process according to the invention, the mixture used is a mixture which contains an azeotropic composition consisting essentially of 1,1,1,3,3-pentafluorobutane and hydrogen fluoride and optionally comprising, in addition, hydrogen chloride, chloro(fluoro)butanes of general formula (I)
CClaF3xe2x88x92aCH2CClbF2xe2x88x92bCH3xe2x80x83xe2x80x83(I)
with a an integer of 0 to 3, b an integer of 0 to 2, the sum of a and b being at least equal to 1, or small amounts of other organic products.
Fundamentally, the thermodynamic state of a fluid is defined by four interdependent variables: the pressure (P), the temperature (T), the composition of the liquid phase (X) and the composition of the gas phase (Y). A true azeotrope is a specific system with two or more constituents for which, at a given temperature and a given pressure, the composition of the liquid phase X is exactly equal to the composition of the gas phase Y. A pseudoazeotrope is a system with two or more constituents for which, at a given temperature and a given pressure, X is substantially equal to Y. In practice, this means that the constituents of such azeotropic and pseudoazeotropic systems cannot easily be separated by distillation.
For the, purposes of the present invention, the expression xe2x80x9cpseudoazeotropic mixturexe2x80x9d means a mixture of two constituents whose boiling point (at a given pressure) differs from the boiling point of the true azeotrope by a maximum of 0.5xc2x0 C. Mixtures whose boiling point differs from the boiling point of the true azeotrope by a maximum of 0.2xc2x0 C. are preferred. Mixtures whose boiling point differs from the boiling point of the true azeotrope by a maximum of 0.1xc2x0 C. are particularly preferred.
At a pressure of 3 bar, the composition of the hydrogen fluoride/1,1,1,3,3-pentafluorobutane azeotropic mixture is about 60/40% by weight, i.e. a hydrogen fluoride/1,1,1,3,3-pentafluorobutane molar ratio of about 11 mol/mol.